Extraction Efficiency Of Light-emitting Diodes Research Articles

Lattice Constant Effect on Diffracted Transmission of ITO Periodic Nanostructures and Improvement of the Light Extraction Efficiency of Light-Emitting Diodes.

We studied the effects of the lattice pitch of indium-doped tin oxide (ITO) periodic nanostructures on the diffracted transmission to improve the light extraction efficiency of light-emitting diodes (LEDs). Periodic hexagonal ITO nanopillars with lattice constants of 600, 800, 1050, 1200, and 1600 nm were fabricated on ITO electrodes. We found that the light extraction efficiency strongly depended on the lattice constant. The LEDs with a lattice constant of 800 nm ITO nanopillars showed an increase in light extraction of 83%. In addition, their electrical properties were not degraded compared to conventional LEDs. The dependence of the extraction efficiency on the lattice constant was also calculated using a 3D finite-difference time-domain (FDTD) method, and this dependence was in good agreement with the experimental measurements. The transmission of each diffraction order and with the total transmission of ITO nanopillars with different lattice constants were calculated using the FDTD method to investigate the enhancement effect.

Enhanced light extraction from AlGaInP-based red light-emitting diodes with photonic crystals.

The photonic crystal (PC) has been demonstrated to be very effective in improving the extraction efficiency of light-emitting diodes (LEDs). In this paper, high-brightness AlGaInP-based vertical LEDs (VLEDs) with surface PC (SPCLED) and embedded PC (EPCLED) were successfully fabricated. Compared with normal LED (NLED), photoluminescence intensities of SPCLED and EPCLED have been improved up to 30% and 60%, respectively. And the reflection patterns of SPCLED and EPCLED were periodic bright points array, showing the ability to control light in PC. Electroluminescent measurements show that three kinds of LEDs have similar threshold voltages. Simultaneously, the light output power (LOP) of SPCLED and EPCLED has been improved up to 24% and 11% at 200 mA, respectively, in comparison to NLEDs. But the LOP decays earlier for EPCLED due to the excessive heat production. Furthermore, it is demonstrated that the SPCLED and EPCLED luminous uniformity is better. This kind of high brightness PCLED is promising in improving the properties of all kinds of LEDs, especially mini LEDs and micro LEDs.

Selective mask formation and gallium nitride template fabrication on patterned sapphire substrates for light-emitting diodes

Patterned sapphire substrates are composed of multiple planes, leading to the undesirable polycrystalline growth of gallium nitride (GaN) during template deposition. However, patterned sapphire substrates and templates with good crystalline quality would be useful for increasing the light extraction efficiency in light-emitting diodes. In this study, a silicon dioxide (SiO2) mask was selectively introduced onto the lenses of a patterned sapphire substrate to suppress the growth of polycrystalline GaN. The SiO2 mask prevented the negative influence of the patterned sapphire substrate on GaN growth under various conditions. High-resolution x-ray diffraction analysis revealed that the GaN template grew as a single crystal in the presence of the SiO2 mask. Furthermore, the compressive stress generated in the GaN template was relieved due to the suppression of polycrystalline growth.

Improved Light Extraction Efficiency of Light-Emitting Diode Grown on Nanoscale-Silicon-Dioxide-Patterned Sapphire Substrate

A conical-shaped Si dioxide nano-pattern was employed to sapphire substrate in order to improve the light extraction efficiency of light-emitting diodes. The conical-shaped Si dioxide nano-patterns were fabricated on a 2-inch sapphire wafer using direct imprinting of hydrogen silsesquioxane material. A blue-LED structure was grown on conical-shaped silicon-dioxide nano-patterned sapphire substrates. Photoluminescence and electroluminescence measurements were used to confirm the effectiveness of the nanoscale Si oxide patterned sapphire. An improvement in the luminescence efficiency was observed when nanoscale Si oxide patterned sapphire substrate was used. 1.5 times higher PL intensity and 1.6 times higher EL intensity were observed for GaN LED structure grown on nanoscale Si oxide patterned sapphire, compared to LED structure grown on conventional flat sapphire wafer.

Chemical etching behavior of non-polar GaN sidewalls

Wet-chemical etching of non-polar GaN films can be applied to form textured surfaces that enhance light extraction efficiency in light-emitting diodes. The etch-induced shapes (trigonal prisms) on the sidewalls of concave and convex mesa patterns defined on a-plane GaN films exhibited an alignment towards the [0001̅] direction. An etch-rate vector model that includes one fast etching direction and two etching directions normal to the fast direction was developed to explain the creation of the etch-induced trigonal prisms. The large lattice parameter along with [0001̅] and single dangling bond of a-plane surface supply enough space for attack of OH− ions, which is confirmed by XPS analysis that indicates the increased hydroxide spectra on a-plane after KOH etching and these are the reason for different etch rate and formation of trigonal prisms.

GaN-based light emitting diodes on nano-hole patterned sapphire substrate prepared by three-beam laser interference lithography

Nano-hole patterned sapphire substrates (NHPSSs) were successfully prepared using a low-cost and high-efficiency approach, which is the laser interference lithography (LIL) combined with reactive ion etching (RIE) and inductively coupled plasma (ICP) techniques. Gallium nitride (GaN)-based light emitting diode (LED) structure was grown on NHPSS by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) measurement was conducted to compare the luminescence efficiency of the GaN-based LED structure grown on NHPSS (NHPSS-LED) and that on unpatterned sapphire substrates (UPSS-LED). Electroluminescence (EL) measurement shows that the output power of NHPSS-LED is 2.3 times as high as that of UPSS-LED with an injection current of 150 mA. Both PL and EL results imply that NHPSS has an advantage in improving the crystalline quality of GaN epilayer and light extraction efficiency of LEDs at the same time.

Optimization of the Cone Pattern in Patterned Sapphire Substrates for the Enhancement of the Light Extraction Efficiency of Light-Emitting Diode

We experimentally investigated the conditions for the enhancement of light extraction efficiency (LEE) in light-emitting diodes (LEDs) with cone patterned-sapphire substrates in terms of a base diameter of the cone pattern (d), pattern-to-pattern pitch (p), and pattern height (h). The LEE increased as both the base diameter of the cone pattern and height increase on the condition of the base diameter of the conepattern being a certain value or lower. However, after the base diameter of the cone pattern exceeded this value, the LEE did not increase as the cone height increased, but the maximum LEE was realized under the optimum condition of the pattern height being equal to half of the base diameter. We present specific numerical values of the patterns on two different conditions of the base diameter of the cone pattern.(Received July 13, 2015)

Study on light extraction efficiency of light-emitting diodes having periodically corrugated enhancement structures with different duty cycles

The duty cycle effect on the light extraction efficiency of light-emitting diodes having different periodically corrugated enhancement structures placed opposite to the emission surface was studied. The experimental results were compared with numerical simulation results. The numerical simulation was performed using a rigorous-coupled-wave-analysis method. In the case of common structures such as a patterned-sapphire structure, we found that, in general, extraction efficiency increased drastically when duty cycle was increased from zero, and then it varied relatively slowly as the duty cycle was increased further; after that, the extraction efficiency dropped drastically as the duty cycle approached 100%. However, for a structure that was composed of autocloned-photonic crystals super-imposing on the periodical corrugation, the efficiency increased monotonically with the duty cycle and reached 83.4% (corresponding to 589% enhancement efficiency) when the duty cycle approached 100%. A method for the fabrication of such high-efficiency structures was proposed.

Light extraction efficiency enhancement using surface-structured light-emitting diodes with a subwavelength coating

The results of our analysis, which was done by using the finite-difference, time-domain (FDTD) methods suggest that surface-structured light-emitting diodes (LEDs) with a subwavelength aluminum-nitride coating will have an enhanced light extraction efficiency. The use of such a LED structure will reduce both the internal reflection and the impedance mismatch, leading to a 3.2-fold enhancement in the light extraction efficiency compared to that for bare planar LEDs. We also present an improved method of modeling the light emission from the LED’s active layers to numerically calculate the light extraction efficiency of LEDs.

Effect of packing density and packing geometry on light extraction of III-nitride light-emitting diodes with microsphere arrays

The finite-difference time-domain method was employed to calculate light extraction efficiency of thin-film flip-chip InGaN/GaN quantum well light-emitting diodes (LEDs) with TiO2 microsphere arrays. The extraction efficiency for LEDs with microsphere arrays was investigated by focusing on the effect of the packing density, packing configuration, and diameter-to-period ratio. The comparison studies revealed the importance of having a hexagonal and close-packed monolayer microsphere array configuration for achieving optimum extraction efficiency, which translated into a 3.6-fold enhancement in light extraction compared to that for a planar LED. This improvement is attributed to the reduced Fresnel reflection and enlarged light escape cone. The engineering of the far-field radiation patterns was also demonstrated by tuning the packing density and packing configuration of the microsphere arrays.

Improvement of the light extraction efficiency of light-emitting diodes based on ZnO nanotubes

Although ZnO nanotubes are emerging as a very promising approach for getting high-brightness light-emitting diodes (LEDs), the light extraction behavior of ZnO nanotubes LEDs has not been reported in detail previously. In this paper, numerical simulations based on finite-difference time-domain (FDTD) method were carried out to investigate the mechanisms through which ZnO nanotube arrays could effectively improve the light extraction efficiency (LEE). The effects of several parameters, including the inner radius, wall thickness, height and period of ZnO nanotubes on improving the LEE were studied. Furthermore, the LEE over the whole visible spectrum has also been investigated, two strong peaks and broadband visible spectrum has been demonstrated. An optimization of ZnO nanotubes geometries with respect to the optical properties has been presented. With the optimized structure, more than four times enhancement of LEE has been achieved. This enhancement is related to the wave-guiding and optical properties of ZnO nanotubes. The optimized structure presented here can be expected as a promising candidate for high efficiency LEDs.

Light extraction enhancement of GaN based light emitting diodes by ZnO nanorod arrays.

This study examined the effects of the alignment of ZnO nanorod arrays (NRAs) on the light extraction enhancement of GaN-based light emitting diodes (LEDs), where ZnO NRAs were synthesized hydrothermally. The shape of the ZnO NRAs was controlled using seed layers for flower and vertical structures. Numerical analysis based on the two-dimensional (2D) finite difference of time domain (FDTD) method showed that the extraction efficiency of LED with bare (without ZnO NRA), vertical ZnO NRA and flower shaped ZnO NRA was 37%, 60% and 49%, respectively. The optical output power of the LEDs with vertical ZnO NRA and flower ZnO NRA was improved by 50% and 30% compared to that of the LED without ZnO NRA at an input current of 100 mA. These results suggest that the vertical alignment of ZnO NRA is important for enhancing the light extraction of GaN based LEDs.

Enhanced Internal Quantum Efficiency and Light Extraction Efficiency of Light-emitting Diodes with Air-gap Photonic Crystal Structure Formed by Tungsten Nano-mask

We demonstrate the blue InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) with an embedded air-gap photonic crystal (PC) which was fabricated by the lateral epitaxial overgrowth of GaN layer on the tungsten (W) nano-masks. The periodic air-gap PC was formed by the chemical reaction of hydrogen with GaN on the W nano-mask. The optical output power of LEDs with an air-gap PC was increased by 26% compared to LEDs without an air-gap PC. The enhanced optical output power was attributed to the improvement in internal quantum efficiency and light extraction efficiency by the air-gap PC embedded in GaN layer.

Angular distribution of polarized spontaneous emissions and its effect on light extraction behavior in InGaN-based light emitting diodes

Angular intensity distributions of differently polarized light sources in multiple quantum wells (MQWs) and their effects on extraction behavior of spontaneous emission from light emitting diode (LED) chips have been studied. Theoretical calculation based on k·p approximation, ray tracing simulation and angular electroluminescence measurement were applied in this work. It is found that the electron-hole recombination in the InGaN MQWs produces a spherical distribution of an s-polarized source and a dumbbell-shaped p-polarized source. Light rays from different polarized sources experience different extraction processes, determining the polarization degree of electro-luminescence and extraction efficiency of LEDs.

Transmittance design of internal reflection triangular-groove grating at large dimension domain

We present that the internal reflective triangular-groove grating can be effectively analyzed and designed by the developed scalar diffraction theory and the effective medium theory at the certain feature size. This grating profile could be applied in enhancing significantly the light extraction efficiency of light-emitting diodes. The accuracy of these both uncomplicated methods is quantitatively evaluated by the comparison of diffraction efficiencies predicted from them to exact results calculated by the rigorous coupled wave analysis method. When the normalized period of a grating is more than fourfold wavelengths of emitted light, the developed scalar diffraction theory is accurate and useful to analyze the internal reflective triangle grating. As the higher order diffraction waves other than zero order wave is not to propagate, the effective medium theory can precisely evaluate the transmittance within the error of about 1.2%. Besides, the used limitation of these simple methods as a function of the normalized height is also quantitatively determined. Furthermore, the transmission characteristics of manufacturing deficiencies of an internal reflection grating as a function of the groove width are quantitatively calculated by using the rigorous coupled wave analysis.

Analysis of the sinusoidal nanopatterning grating structure

To accurately utilize the sinusoidal nanopatterning grating structure for greatly improving the light extraction efficiency of light-emitting diodes, the transmittance characteristics of these nanostructures as a function of the normalized period and the normalized depth are investigated quantitatively. It is found that when the normalized period of nanostructure is less than 0.46; in other words, only zeroth order transmission light is propagating, the light extraction efficiency can be enhanced immensely due to the higher transmittance of surface structure. When the period scale of nanostructure is more than the emitted wavelength, the transmittances decrease as the structure height increase. However, as the period dimension is less than the emitted wavelength, the transmittance of surface profile is increased as the depth increases. Besides, in order to easily analyze and effectively design the transmittance characteristics of these nanopatterning structures integrated in LEDs, we propose that the developed scalar method and the effective medium theory can be used accurately. The accuracy of both uncomplicated methods is quantitatively evaluated by the comparison of diffraction efficiencies predicted by the scalar theory and effective medium method, to exact results calculated by the rigorous coupled wave analysis method.

Micro/nano structures induced by femtosecond laser to enhance light extraction of GaN-based LEDs

Surface texturing has been widely adopted to enhance the light extraction efficiency of light-emitting diodes (LEDs), and chemical etching is a technique commonly used to produce surface texturing. This study employed femtosecond lasers to apply ITO films directly onto the surface of LEDs to generate periodic micro/nanostructures and roughen the surface without contact or chemical substances. As a result, photons emitted in the active region escape into the free space, due to the scattering effect produced by texturing. This study discovered that light-emitting efficiency increases with surface roughness, and achieved an improvement of 18%. Caution regarding laser fluence was required during laser processing to avoid damaging the LED beneath the ITO film, which could detract from the electrical characteristics.

Optimizing textured structures possessing both optical gradient and diffraction properties to increase the extraction efficiency of light-emitting diodes

The external quantum efficiency of a light-emitting diode (LED) is strictly limited by total internal reflection and Fresnel reflection effects. In this study, we sought to optimize light extraction by monitoring the shape effects of four kinds of periodic textured structures (nanorod, inverted rod, pyramid, inverted pyramid) on the surface of gallium nitride (GaN)-based LEDs. We employed the three-dimensional rigorous coupled waves approach to calculate the direct emissions at different incident angles on the various textured structures, and then determined an optimized structure that would improve the extraction efficiency of LEDs. The optical gradient of the inverted pyramid structure could decrease not only the Fresnel reflection at incident angles less than the critical angle but also the total internal reflection at incident angles greater than the critical angle. Many inverted pyramid structures at the GaN–air interface, with various sizes and periods, provided enhancement factors of greater than 150%.

Double-Grating Displacement Structure for Improving the Light Extraction Efficiency of LEDs

To improve the light extraction efficiency of light-emitting diodes (LEDs), grating patterns were etched on GaN and silver film surfaces. The grating-patterned surface etching enabled the establishment of an LED model with a double-grating displacement structure that is based on the surface plasmon resonance principle. A numerical simulation was conducted using the finite difference time domain method. The influence of different grating periods for GaN surface and silver film thickness on light extraction efficiency was analyzed. The light extraction efficiency of LEDs was highest when the grating period satisfied grating coupling conditions. The wavelength of the highest value was also close to the light wavelength of the medium. The plasmon resonance frequencies on both sides of the silver film were affected by silver film thickness. With increasing film thickness, plasmon resonance frequency tended toward the same value and light extraction efficiency reached its maximum. When the grating period for the GaN surface was 365 nm and the silver film thickness was 390 nm, light extraction efficiency reached a maximum of 55%.

Study on the light extraction efficiency of GaN-based light emitting diode by using the defects of the photonic crystals

Many studies have shown that the photonic crystals and surface nanostructures can improve the light extraction efficiency of light emitting diode (LED). The defects and disorders exist in photonic crystals inevitably as the manufacturing technology limitations, and so the light extraction efficiency of LED will be changed correspondingly. In this work we perform a simulation study on defects and disorder of photonic crystals by the finite-difference time-domain (FDTD) method, and we speed up the FDTD by GPU acceleration technology. Simulation results show that a small number of defects in photonic crystals LED do not reduce the LED light extraction efficiency. Instead, the light extraction efficiency of LED will be increased by adding some defects into the photonic crystals. We give a detailed analysis of its physical mechanism. We design a kind of photonic crystals with defects, and its light extraction efficiency achieves 1.6 times that of the perfect photonic crystals.